Retrieving hydrocarbons from subterranean reservoirs is becoming more difficult, as existing reserves are depleted and production becomes more expensive. It has been estimated that mature fields account for up to 70% of the world's production or more. In order to increase production, reservoirs are often hydraulically fractured to stimulate production of hydrocarbons from the wellbore. Hydraulic fractures are created in subterranean formations by hydraulically injecting water or high viscosity fluid (also referred to as fracturing fluid) containing a proppant at a high flow rate into a wellbore and forcing the fracturing fluid against the formation strata by pressure. The formation strata or rock is forced to crack, creating or enlarging one or more fractures. The proppant subsequently prevents the fracture from closing completely and thus provides improved flow of recoverable fluid, i.e., oil, gas or water.
Because aging wells often produce from multiple intervals, some very thin, the ability to locate these stimulation treatments with pinpoint accuracy is a key to more effective remediation and increased ultimate recovery. Also in more “non-conventional” plays like the fractured shales, the quality and extent of the fracture job is paramount to the financial success of the well and the play. However, few methods exist for accurately visualizing fracture length, proppant penetration, and estimated flow in the new fracture are required in order to accurately assess production capabilities and the need for further remediation before production is initiated.
Numerous techniques exist for detecting the fracture geometry of a well using various imaging techniques. For example, Hocking et al., U.S. Pat. No. 6,330,914, provides a method for monitoring a propagating vertical fracture in a formation by injecting conductive fracture fluid into the formation to initiate and propagate the fracture; energizing the fracture fluid via an electrical voltage while the fracture propagates; and measuring the inducted electromagnetic field parameters to judge about the fracture development and geometry. Further, McCarthy, et al., WO2007013883, provides introducing a target proppant; transmitting electromagnetic radiation from about 300 megahertz-100 gigahertz; and analyzing a reflected signal from the target particle to determine fracture geometry. Lastly, Nguyen et al., U.S. Pat. No. 7,073,581, describes electro-conductive proppant compositions and related methods of obtaining data from a portion of a subterranean formation. These techniques focus on detecting data utilizing a series of geophones connected to conventional seismic equipment, which converts ground movement, i.e., displacement, into voltage.
Additionally, fractures can be monitored and approximately mapped three-dimensionally during the fracturing process by a micro-seismic technique. The micro-seismic technique detects sonic signatures from rocks cracking during the fracturing process. The setup of this technique is prohibitively expensive to do routinely, and the data that is generated tends to be relatively inaccurate due to high background noise. Further, the process can only be performed during the fracturing process and cannot be repeated thereafter.
Although these techniques yield useful information, its usefulness is limited to fracture locations near the wellbore and yields little if any useful information relating to the dimensions of the fracture as it extends into the formation. Therefore, a need exists for monitoring and mapping fractures as they extend away from the oil or gas well.